Tomosynthesis with shifting focal spot and oscillating collimator blades

ABSTRACT

In a tomosynthesis system a static focal spot is moved in a direction opposite to and generally synchronized with the directional movement of an x-ray source and X-ray collimator blades are moved during each exposure in synchronization with the shifting of the static focal spot. The synchronized movement of the static focal spot, x-ray tube and collimator blades helps keep the effective focal spot fixed in space relative to the breast, detector or both during the entire duration of the exposure and keeps the x-ray field on the detector and breast static. The shifting collimator blades follow an oscillating pattern over the multiple x-ray exposures of a tomosynthesis scan.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 12/849,294 filed Aug. 3, 2010, which is acontinuation-in-part of U.S. patent application Ser. No. 12/623,472filed Nov. 23, 2009, which claims priority to U.S. Patent ProvisionalApplication Ser. No. 61/117,453 filed Nov. 24, 2008, all of which areincorporated by reference herein.

BACKGROUND OF THE INVENTION

Breast tomosynthesis is a three-dimensional imaging technology in whichimages of a stationary compressed breast are acquired at multiple anglesduring a short scan. The images are organized as a series of thinhigh-resolution slices that can be displayed individually or in adynamic cine mode. Breast tomosynthesis systems are similar tomammography systems except that the x-ray source is moved to a varietyof different imaging positions during image acquisition. Reconstructedtomosynthesis slices advantageously reduce or eliminate problems causedby tissue overlap and structure noise in single slice two-dimensionalmammography imaging. Digital tomosynthesis, which combines digital imagecapture and processing with simple tube/detector motion as used incomputed tomography (CT) but over a smaller rotational angle than thatused in CT, offers the additional possible advantages of reduced breastcompression, improved diagnostic and screening accuracy, fewer recalls,and 3D lesion localization. However, movement of the x-ray sourceintroduces some technological complications.

Typical tomosynthesis systems are arranged to smoothly and continuouslytraverse a path during an image scan because stop-and-start scanningprocedures tend to reduce image quality. The x-ray source is activatedfor an exposure time of about 10 ms to 100 ms as the x-ray source movesinto each of several imaging locations in the imaging path, and exposureis repeated with a cycle period of 200 ms to 2 seconds. After eachexposure the x-ray source is deactivated. As the x-ray source movesbetween imaging locations the contents of the digital image detector areread out and stored. There is a minimum time period associated withreading the image from the digital detector, and the overall speed ofthe tomosynthesis scan is determined by the minimum time period fordetector read, the exposure time at each location and the number ofexposures. As the x-ray source is continuously moved through spaceduring each exposure period in a tomosynthesis system the focal spotalso moves. The resultant focal spot movement causes image blurring andreduces diagnostic accuracy.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, an apparatus comprises:an x-ray source which defines a static focal spot; a collimator whichcontrols the dispersion of radiation from the x-ray source; a detectorwhich obtains images while the x-ray source is in motion; and a motioncontroller which synchronizes movement of the static focal spot, x-raysource and collimator such that the static focal spot and collimatorsare moved in a direction opposite to directional movement of the x-raysource during an exposure period.

In accordance with another aspect of the invention, a method comprises:performing a tomosynthesis scan including synchronizing movement of astatic focal spot, x-ray source and collimator using a motioncontroller, including moving the static focal spot and collimator in adirection opposite to directional movement of an x-ray source during anexposure period.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a breast tomosynthesis system.

FIGS. 2 and 3 illustrate synchronized movement of the static focal spot,x-ray tube and collimator blades.

FIG. 4 illustrates an x-ray tube with a focal spot and collimator bladeposition controller.

FIG. 5 illustrates a process of using a tomosynthesis system for 2D and3D imaging.

DETAILED DESCRIPTION

FIG. 1 illustrates a tomosynthesis system 100 which includes an x-raytube 110, upper and lower compression paddles 130, 135, an anti-scattergrid 140 and a detector 160. The x-ray tube 110 includes a cathode 112,an anode 114 that is mounted on a shaft 116 and rotated by a motor 118,and a tube port 120. Also shown attached to the x-ray tube are a filter122 and collimating means such as collimator blades 124.

The illustrated x-ray tube is a glass vacuum tube. Within the cathode112 is a heated filament. When the x-ray tube is turned ‘on,’ a currentis passed through the filament, thereby heating the filament and causinghigh energy electrons to be dislodged from the filament. A high voltagebetween cathode and anode causes the electrons to accelerate toward atarget location 125 on the anode. The anode is made, for example, fromtungsten and is rotated by motor 118 to avoid local overheating of thetarget location 125 on the anode. Electrons are focused to a specifictarget location by means of a focusing cup (not shown) which is aseparate control electrode that is cylindrical in shape and attached tothe cathode, partially surrounding a filament of the cathode. Thedislodged electrons collide with the tungsten atoms of the anode andx-ray photons are generated having bremsstrahlung radiation andcharacteristic line emission spectra. The x-ray photons are emitted inall directions from the target location 125.

The x-ray photons which come out of the tube port 120 are used forimaging. For the purposes of this application, the x-ray photons whichcome out of the tube port define a static focal spot 127. The staticfocal spot 127 is the focal spot as it appears from directly beneath thex-ray tube from the perspective of the breast, at or near the chestwallposition of the patient. Focal spot characteristics are defined byInternational Standard CEI IEC 60336. Generally, the focal spot isrectangular in shape and stated for two normal directions of evaluationreferred to as the length and width direction. The length direction isgenerally parallel to a longitudinal axis of the x-ray system, and thewidth direction is generally perpendicular to the longitudinal axis. Thelongitudinal axis of an exemplary tomosynthesis system is shown in FIG.1.

Static focal spot size refers to the focal spot size at any giveninstantaneous moment in time, as compared to the time-averaged focalspot size during an x-ray exposure of finite time period which isreferred to herein as the effective focal spot size of an x-rayexposure. The size of the static focal spot 127 significantly affectsthe heat loading capacity of the x-ray tube. Greater heat loading ispossible with larger focal spots, thereby allowing a higher tube currentmA to be safely provided. The size of the focal spot is determined by acombination of factors including the size and shape of the filament andthe shape and bias voltage of the focusing cup. The angle of the targetsurface further defines a focal spot size along the so-called lengthdirection.

The size of the focal spot is an important factor in a diagnostic x-raytube because it affects the resolution of the radiography system. Moreparticularly, systems having smaller focal spots have better resolution,so reducing static focal spot size is one design goal. For example,mammography systems may be designed to provide a 0.3 mm focal spot forimaging (0.1 mm focal spot for high magnification images). Movement ofthe x-ray source during image exposure effectively stretches the widthof the static focal spot, resulting in an effective focal spot which iswider than the static focal spot and which decreases image sharpness.The size of the effective focal spot is therefore determined by the sizeof the static focal spot and the motion of the static focal spot duringexposure, and the effective focal spot (aka dynamic focal spot) is theaccumulation of the static focal spot over time.

As illustrated in FIGS. 2 and 3, the static focal spot is moved at thesame linear speed in a direction opposite to and generally synchronizedwith the directional movement of the x-ray source during the exposureperiod. Further, the x-ray collimator blades 124 are moved during eachexposure in synchronization with the movement of the static focal spotto keep the collimated rays contained within a boundary as governed byFDA field limitation compliance regulation. The synchronized movement ofthe static focal spot, x-ray tube and collimator blades helps keep theeffective focal spot fixed in space relative to the breast for theentire duration of the exposure and keeps the x-ray field on thedetector and breast static.

The focal spot and shifting collimator blades follow a linearoscillating pattern over the multiple x-ray exposures of a tomosynthesisscan. Before an exposure the focal spot and collimator blades are movedto start positions. The collimator blades then shift following themotion of the static focal spot during the exposure. At the end ofexposure the focal spot and collimator blades are moved back to thestart positions to prepare for the next exposure. This process isrepeated until all x-ray exposures are finished in a scan. When the scanis complete the focal spot and collimator blades are set to a centerposition, which is the position for conventional imaging.

FIG. 4 illustrates an x-ray tube 110 including a vacuum tube 400 whichencases an anode 114, a cathode 112 and an anode rotor 410. Thecollimator blades and tube (c'arm) each have closed loop controllerswhich are calibrated for positional accuracy. A main processor 162(FIG. 1) equipped with memory for storing program code regulates themotion synchronicity of the focal spot, collimator blades and tube(c'arm). According to one aspect of the invention, the x-ray tubefurther includes a focal spot and collimator position controller 600.The controller may be coupled to the cathode 112 to deflect the electrontrajectory in the ‘width’ direction. In its simplest form the controllercomprises two parallel metal plates located next to the focusing cup610, with a bias voltage applied across the plates that can shiftelectron motion direction, and therefore the target location on theanode. Focal spot displacement is proportional to the bias voltage levelapplied by a voltage controller 450. The shift of the focal spot istherefore controlled via an application of a bias voltage across theplates. In several embodiments, the bias voltage can be dynamically orstatically configured prior to x-ray exposure.

Referring now to FIG. 5, a process of using a tomosynthesis system for2D and 3D imaging will now be described. At step 510 the tube (C'arm) ismoved CCW (counter clock-wise) X.X° to a start position. The focal spotand collimators blades are moved in a direction opposite that of thetube to a start position. As indicated by step 512, the tube (C'arm)starts tomosynthesis motion in the CW (clock-wise) direction and thestatic focal spot is moved in a direction opposite to and generallysynchronized with the directional movement of the x-ray source, and thex-ray collimator blades are moved in synchronization with the shiftingof the static focal spot. Although illustrated as a step in the processit will be appreciated that these synchronized movements are continuousover the duration of the exposure. The x-ray tube is activated uponreaching an initial imaging position as illustrated by step 514. In oneembodiment, each exposure takes less than 60 ms. During the exposure,the gantry continues to move towards the +7.5 degree position, and thex-ray tube focal spot motion controller sets the focal spot to astarting position on the anode which is pre-calculated based on thex-ray technique and gantry scan speed of the intended tomosynthesisscan, moves the static focal spot in the opposite direction (in thisexample, clockwise tomosynthesis scan). At step 516, when the exposureis complete and focal spot at the same time reaches the pre-calculatedstop position, the x-ray tube is turned off and the static focal spotand collimator blades are moved to the start position for the nextexposure. At step 518 it is determined whether the end point of theclock wise scan has been reached (gantry at the +7.5 degree position).If not, steps 512 through 516 are repeated until all tomosynthesisprojection images are obtained. At step 520 the tube (C'arm) is moved toa zero position (which is typically 0°) and the focal spot andcollimator blades are moved to a center position to prepare for aconventional mammographic exposure. If the focal spot size had beenincreased for tomosynthesis imaging, it is reduced to the range whichprovides desired mammogram resolution. At step 522 the image is obtainedand the process is complete.

It should be noted that although the x-ray tube is described as beingturned ‘on’ or ‘off,’ some systems have x-ray tubes that arecontinuously on during the scan, with image capture being controlled bycapture of the x-rays at the detector at select ‘exposure times’ timesduring the scan. In such instances, it can be appreciated that the focalspot motion is synchronized to the exposure start and exposure endtimes, regardless of whether the x-ray tube is cycled or is continuously‘on.’

Although a system, method and process of the present invention has beenshown and described to improve tomosynthesis image clarity by static ordynamic management of focal spot size and position during an x-rayexposure, it should be noted that the present invention is not limitedfor use to any particular imaging modality. Rather it is envisioned thatthe x-ray tubes and collimator blades of the present invention may haveutility in any system which obtains images while an x-ray source is inmotion. For example, computed tomography (CT) systems experience focalspot blurring. The modified x-ray tube and collimator blades of thepresent invention may advantageously be used with CT systems to reducethe FS blur, making the Modulation Transfer Function (MTF) across fieldof view isotropic. In a breast CT system, one benefit of such animprovement would be that the MTF at the breast edge would be as good asthat in the breast center in the horizontal plane. Accordingly, theembodiments described above are intended to be examples and are notintended to be exhaustive or to unduly limit the claimed inventions. Theexamples are intended to describe principles that persons skilled in theart may use to practice the claimed inventions, using variations andmodifications of the disclosed examples that are suited to a particularenvironment. It is intended that the scope of the invention be definedby the appended claims and their equivalents.

What is claimed is:
 1. A system for tomosynthesis imaging comprising: anx-ray source configured to move to different imaging positions duringtomosynthesis image acquisition; a detector configured to obtain imageswhile the x-ray source is in motion; and a collimator configured tocontrol dispersion of radiation from the x-ray source, the collimatorbeing positionable so that the collimator moves in a direction oppositeto the directional movement of the x-ray source during tomosynthesisimage acquisition.
 2. The system of claim 1, wherein the collimatorcomprises two blades, one blade spaced apart from a second blade.
 3. Thesystem of claim 2, wherein a distance between the two blades is constanteven when the collimator moves during tomosynthesis image acquisition.4. The system of claim 1, wherein the collimator moves to differentpositions relative to the x-ray source, the collimator positioned at astart position before an exposure period, the collimator moves in thedirection opposite to the directional movement of the x-ray sourceduring the exposure period, and the collimator positioned back at thestart position after the exposure period.
 5. The system of claim 4,wherein the collimator is positioned at a center position when thetomosynthesis image acquisition is complete.
 6. The system of claim 4,wherein the system is also configured to obtain conventional 2Dmammographic images, the collimator positioned at a center position toobtain the conventional 2D mammographic images.
 7. The system of claim 1further comprising a motion controller to synchronize movement of thex-ray source and the collimator.
 8. The system of claim 7 furthercomprising a main processor to regulate the movement of the collimatorand the x-ray source.
 9. The system of claim 1, wherein the collimatorand the x-ray source each have closed loop controllers.
 10. The systemof claim 1 further comprising a collimator position controllerconfigured to control the position of the collimator.
 11. The system ofclaim 10, wherein the collimator position controller comprises twoparallel metal plates located next to a focusing cup, which focuseselectrons of the x-ray source to a specific target location.
 12. Thesystem of claim 1, wherein the x-ray source defines a static focal spotand the static focal spot moves in a direction opposite to directionalmovement of the x-ray source during tomosynthesis acquisition.
 13. Thesystem of claim 12, wherein the static focal spot is moved at a linearspeed equal to that of the x-ray source during an exposure period. 14.The system of claim 12, wherein the static focal spot and the collimatorfollow a linear oscillating pattern over multiple x-ray exposures duringtomosynthesis image acquisition.
 15. The system of claim 12, whereinstatic focal spot displacement is proportional to a bias voltage levelapplied by a voltage controller.
 16. The system of claim 15, wherein thebias voltage is dynamically or statically configured prior to anexposure period.
 17. The system of claim 12, wherein the static focalspot moves to different positions relative to the x-ray source, thestatic focal spot positioned at a start position before an exposureperiod, the static focal spot moves in the direction opposite to thedirectional movement of the x-ray source during the exposure period, andthe static focal spot positioned back at the start position after theexposure period.
 18. The system of claim 17, wherein the static focalspot is positioned at a center position when the tomosynthesis imageacquisition is complete.
 19. The system of claim 17, wherein the systemis also configured to obtain conventional 2D mammographic images, thestatic focal spot positioned at a center position to obtain theconventional 2D mammographic images.